The invention relates to an apparatus for straining suspensions, especially fibrous suspensions, including a housing, a rotationally symmetric screen basket and coaxially rotating purging blades for sweeping retained objects from the surface of the screen basket. The apparatus further includes an inlet for the unpurified liquid suspension and a drain for the purified suspension, as well as an outlet at the downstream end of the screen basket for releasing the screened-out impurities. Straining apparatus of this general construction, of both open and closed type, is used especially in the paper industry for removing contaminants or foreign objects from fibrous suspensions. A suspension of cellulose and wood pulp may contain splinters, fiber bundles, sand and the like and when old paper is recycled, pieces of foil and styrofoam, splinters and the like may have to be removed from the suspension. The suspension to be purified is usually admitted at one end of the strainer. When the straining takes place under pressure, the suspension fills the entire volume of the strainer. When the straining takes place without pressure, in an apparatus where the suspension flows from the inside to the outside, i.e., in a straining apparatus where the suspension flows under centrifugal force, the centrifugal effect of the purging blades causes the formation of a fluid layer of a certain thickness on the inside of the screen basket. Strainers of the type described above can, however, also use flows which proceed from the exterior toward the interior of the apparatus.
The contaminants trapped by the screen are removed from the strainer at the downstream end of the screen basket either in free outflow or with the aid of a control member, for example, a shut-off valve. Partly, because of gravity, and partly due to the effects of the purging blades which are usually disposed slightly helically, the contaminants are gradually transported toward the outlet. The rotation of the purging blades generates positive and negative-going pressure pulses, each of which transports the contaminants a small distance toward the end of the screen basket.
One disadvantage of this apparatus, especially in larger strainers is the risk that contaminants which are deposited near the inlet of the screen may still find their way through the screen while they migrate to the outlet. This could happen if they are further comminuted or if, for example, elongated splinters assume a position for passing through the holes in the screen.